In the ordinary decarburization method in which oxygen is blown into molten steel, the resulting reaction product carbon monoxide (CO), produced by the decarburizing reaction in accordance with the following formula (1), is a so-called incomplete combustion product wherein the heat of chemical reaction that carbon (C) as a fuel possesses is actually utilized to the extent of at most one third of the maximum: EQU C+1/2O.sub.2 =CO+26.4 Kcal/mol (1)
In a case wherein C is satisfactorily burnt, i.e., good combustion takes place as shown in the following formula (2), to produce carbon dioxide (CO.sub.2), the reaction heat produced there reaches as much as 94.05 kcal/mol: EQU C+O.sub.2 =CO.sub.2 +94.05 kcal/mol (2)
The effective recovery of the heat of chemical reaction concealed in this CO, that is latently held therein, for utilizing it positively has been attempted for many years by many persons. The Kaldo process and the Rotor process, which are employed in refining pig iron into plain carbon steel, out of those many propositions, adopt a method of blowing oxygen into the furnace in order to directly burn the CO in question. Another method is to lead the CO out of the furnace for recovering and utilizing it, such as by the recovery of exhaust gas in a converter. In the former two methods, wherein combustion takes place inside the furnace, the furnace body must be turned or rotated, and the heat generated when CO is converted to CO.sub.2 must be stored in the furnace wall for transmittal to the molten steel indirectly. These methods are consequently obliged to suffer some disadvantages enumerated as follows, and they are therefore regarded at present as being obsolete:
(1) a huge driving apparatus must be installed for turning the converter, or a revolving furnace body must be provided, and PA1 (2) vibration of the furnace body occurring when it is turned, and exposure of the furnace wall to high temperature greatly increases the wear of refractories in the furnace.
For the refining of special steels such as stainless steel which contains chromium, an extremely low percentage of carbon, for example 0.01% is required, in comparison to plain carbon steel where as high a percentage of carbon as 0.45% is allowed. Thorough decarburization to the molten steel which is obtained in an electric furnace, etc., must be carried out in the refining of stainless steel. According to general belief, the oxygen blown into the bottom of a furnace in the decarburization of such chromium-containing steel is first turned to an oxide, for example of chromium, before being finally converted to CO according to the process shown in the formula (3): EQU 3Cr+20.sub.2 .fwdarw.Cr.sub.3 O.sub.4 EQU Cr.sub.3 O.sub.4 +4C.fwdarw.3Cr+4CO (3)
As a method effectively functioning in such a decarburization system for molten steel, a type called the AOD (Argon Oxygen Decarburization) process has been developed and practiced in recent years, wherein a gas mixture of argon (Ar) and oxygen (O.sub.2) is blown from below into the upper surface level of the molten steel. This type of method is problematical, irrespective of its feature of accelerating decarburization by restraining oxidizing of chromium, in increasing the wear of the tuyere due to blowing of the oxygen-containing gas therethrough and incapability of speeding-up the oxygen supply even at a peak of decarburizing action. As a substitute for this AOD process, another process called "Oxygen top-blowing--Argon bottom-blowing" was developed, wherein O.sub.2 and Ar are respectively blown from individually separated sources, the former being blown into the upper surface level of the molten steel and the latter below the upper surface level thereof. This process was recognized to be as effective as the previous AOD process in the refining of steel. Both of those two processes are effective, however, only in turning C to CO, but still unsatisfactory in the utilization of the heat of chemical reaction according to the following formula (4): EQU CO+1/2O.sub.2 =CO.sub.2 30 67.7 kcal/mol (4)
In such processes wherein C is turned CO by way of oxides of chromium, residue of the oxides of chromium forms slag floating on the surface of the molten steel. Throwing away of such slag is disadvantageous not only in varying to chromium content in the finished steel, but also in increasing loss of precious chromium. For preventing this disadvantage, addition of silicon (Si), a reducing agent for the slag, has been practiced. Then the chromium can be returned to the molten steel according to the following chemical reaction shown in the formula (5). EQU 2Si+Cr.sub.3 O.sub.4 .fwdarw.2SiO.sub.2 +3Cr (5)
Such addition of Si is effective indeed in returning the chromium again into the molten steel, so economically preventing loss of chromium, but the Si is obliged to thrown away with the slag as an inevitable loss. It is therefore highly desirable to minimize the amount of the Si added in the refining process, as a matter of course.